before a baby isborn, there are a lot of adaptations that wesee that allow the baby to take nutrientsand oxygen from mom and successfullyget those nutrients and oxygen to the differentcells that need them in the body. so what i wanted todo is kind of draw out for you in one diagramall the kind of things that we see before birth.
these are all the thingsthat are happening while the baby isstill in the uterus. sometimes we say in utero. so before birth,what do we notice? well, this structure overhere, this is our placenta. this is partially momand partially fetus. so the placenta has mom's bloodkind of pooling in this area. and the baby actually sticksits little capillaries inside of that pool of blood.
and you can see thatthe purplish blood is kind of going in, and thereddish blood is coming out. and essentiallywhat i was trying to draw there is thatoxygen is getting picked up. so it's actuallygetting oxygenated. and this blood, asit's kind of reddish, is joining into thisblood vessel down here. so this is kind of thesmiley part of our face. and this is our umbilical vein.
so this umbilicalvein is actually going to carry oxygen and bloodback towards the liver area. so let me actually just jotthat name down, umbilical vein. and this is actually thefirst of the adaptations i was talking about. so i'm going to make a littlelist of adaptations over here on the side, just so we cankeep track of what they are. and the first one willbe the umbilical vein. so once the blood goesinto the umbilical vein,
it has kind of a branch point. you can see that it can eithergo to the right or the left. and if it goes to the left,it's going to enter the liver. so if it goes kind of this way,it's going enter the liver. and it's going to takea while for that blood to come out on theother side, because it has to go through all thelittle capillaries in the liver and then emergeon the other side. but there is a shortcut.
so the shortcut-- letme just circle it here. the shortcut is actuallygoing to be right here. so let me just make sureit's very clear what the shortcut is. this is called ourductus venosus. and the ductusvenosus is basically going to allow blood to go fromthe umbilical vein, through it. so it's like a little tube. so it is just like anyother blood vessel.
it's going to go through it. and on the other side,it hits and meets up with our inferior vena cava. so this is ourinferior vena cava. i'll write ivc just for short. and the ivc or theinferior vena cava is a large vein picking upblood from the right leg and also from the left leg. so this is ourleft leg down here.
so the interiorvena cava meets up with the blood coming fromthe umbilical vein, which is very oxygenated. and so this blood i'm goingto draw is kind of purplish now, because it's kindof got some oxygen, but it's not as rich as whatwas coming out initially from the umbilical vein becauseit mixed in with the ivc. and that blood dumpsinto the right atrium. so this is our rightatrium on this side.
and simultaneously,you actually have blood from the superiorvena cava, or svc. this is our head and armregion, draining down this way. and this blood also kind ofends up in the right atrium. so you've got thisblood kind of mixing. and now i'm going to draw itas kind of a deeper purple, because it's mixed up blood. now, the secondadaptation, then-- let me just make sure idon't skip out on these.
this is the first one. the second one wouldbe the ductus venosus, that i wrote out. which is, as i said,kind of a shortcut from the umbilical vein overto the inferior vena cava. now, the blood isin the right atrium. so it has a couple of options. first, it could simply godown into the right ventricle. and some of the blood does that.
it just goes right downinto the right ventricle. and if goes intothe right ventricle, it's going to get squeezed. and once it gets squeezed, itgoes into the pulmonary artery. this is my pulmonaryartery over here. and we know the pulmonaryartery has a branch over to the lungs on both sides. so we've got some blood goingto one lung and some blood going to this other lung.
but remember, once that bloodkind of approaches the lungs, we have to think about what'sgoing on inside of the lungs. so let me draw outwhat's happening then inside the lungs. you've got thesesacs, air sacs, that actually are not full of air. right? because when the baby isstill inside of the uterus, or when the fetus is in theuterus, it's full of fluid.
so you've got thesesacs full of fluid. and going past them arelittle blood vessels. so this is a littleblood vessel. and let's say this isan arteriole over here. now, if it's full of fluid, thatmeans there's not much oxygen. so what ends uphappening is that there's a process called hypoxicpulmonary vasoconstriction. and what that means isthat the alveolus literally tries to help constrictthe arteriole.
so the arteriole has somesmooth muscle like this. and because there's nooxygen, the alveolus is going to cause that littlearteriole to basically contract down. so basically, it looks alittle bit more like this. and when it looks like that,what we've essentially done is increased the resistanceof that arteriole. and if this is happeningmillions of times in millions andmillions of alveoli,
then the entire lung is goingto have a lot of resistance. a lot of resistance inthe lung at this point. so if that's the case, ifthere's a lot of resistance, then a few things we haveto kind of deduce from that. the first is that ifthere's a lot of resistance, then the pressure in thepulmonary artery-- remember, this is our pulmonary arteryright here, these two. i'll actually draw a littlearrow to both of them. the pressure in thepulmonary artery
is going to go very high. so these pressuresare going to be high. and that's simply becauseyou've got a lot of resistance that you have to tryto fight against. so they have a lot of pressure. and if there's a lot of pressurein the pulmonary artery, just think back,and think, well, where did that pulmonaryartery come from? it came from theright ventricle.
so for there to beforward flow of blood, you better havea lot of pressure in the right ventricle. and then i could take theargument back and say, well, if you have a lot of pressurein the right ventricle, then you must have a lot ofpressure in the right atrium. so you have a lot ofpressure, basically, on the right side ofthe heart, because of the fact that you've got alot of resistance in the lungs.
so these pressures, especiallythe right atrial pressure, starts getting so high thatit starts getting higher than the pressurein the left atrium. and so you get a littlebit of blood flow that starts goingfrom the right atrium, across that foramen ovale,that allows-- right here-- that allows blood toactually go across it. so this is our foramen ovale. foramen ovale allows bloodto go from one atrium
over to the other. and since bloodcan now go across, you're going to seesome of the blood continue down inthe right ventricle. but some of the blood will alsokind of go across into here. and that's actuallyquite useful, because at the same time thatyou have blood going across, you actually don't havetoo much blood coming back through the pulmonary veins.
and the reason forthat, again, is because it's hard to getblood flow through the lungs because there's somuch resistance there. so you have a little bitof blood kind of coming in and you get some bloodcoming from the right atrium. now, from the leftatrium, blood is going to go down intothe left ventricle. and on its going to getsqueezed around into the aorta. so now you getblood in the aorta.
that gets squeezed there orsent there from the aorta-- or from the left ventricle. i apologize. so the left ventricleis squeezing blood down into the aorta. and the aorta is distributingblood all the way down. now, before i finishoff showing you where the aortic blood goes,let's actually make sure i don't forget mylist over here.
my third adaptation, then,should be the foramen ovale, foramen ovale sendingblood from the right atrium to the left atrium. and a fourth adaptation,actually, i've just kind of sketched out, buti haven't talked about yet, is right here. so you actually have thislittle guy right here. a little connection,a little vessel-- you can think of it as avessel because blood flows
through there-- between thepulmonary artery and the aorta. so this thing right here iscalled the ductus arteriosus. so the ductusarteriosus allows blood to go from the pulmonaryartery to the aorta. and why would blood go inthat direction in particular? well, remember thepulmonary artery, again, has very high pressures. and the high pressures arebecause of the high resistance in the lungs.
so because of those highpressures, blood, of course, goes from highpressure to a place where there's lowerpressure usually. and in this case, it's goingto go from the pulmonary artery over to the aorta. so it's actually going toflow in this direction. let me just draw a little arrow. it's going to go flowingin that direction. so ductus arteriosus isanother fetal adaptation.
so we've got four so far. ductus arteriosus. and this actuallyexplains, then, why you don't get toomuch blood coming back because a lot of the bloodgoes into the pulmonary artery trunk, ends up goinginto the aorta. it actually doesn'teven go into the lungs because theresistance is so high. so now let's kindof wrap this up.
let's say blood isnow down in the aorta. as i said, it's goingto go into the legs. and it's alsogoing to kind of go into these internaliliac arteries. so i've drawn thesearteries here. these are the internaliliac arteries. and there are, ofcourse, lots of branches off the internal iliac. but the important branch thati want to point out right now
is this one. this branch, thismajor one that i'm kind of sketching in,this is actually-- we have a name for it. we call this theumbilical artery. so this is actuallybringing blood back towards the placenta. now, why would so muchblood go to the placenta? i mean, that's a fair question.
why doesn't it go--there's a branch here that goes to the bladder. there's a branch thatgoes to other places. why is blood going intothe placental branch or the umbilical artery? why so much? well, it turns out thatthe placenta-- and this is very clever-- actuallyhas a very low resistance, very low resistance.
so just as the lungshave a high resistance and they're kind of makingblood divert away from them, the placenta hasa low resistance, and it makes blooddivert towards it. so you can see now that thisis a really ingenious kind of system. we have these five adaptations--the umbilical vein, the umbilical artery now,we have the ductus venosus, and we have the foramen ovale,and the ductus arteriosus.
i don't want to missout on any of them. so we have five importantadaptations here. and this is how bloodflows in the fetus.
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